It is common to provide charts of various types, such as topographical charts or depth charts for bodies of water, that define contour lines representing the topography. A given contour line is the locus of points that have the same value of the pertinent parameter. For example, in the case of a topographic land chart, a contour line represents a constant elevation above sea level, or in the case of a nautical chart, a contour line represents a constant depth below the water surface. Alternatively, charts can employ colors for defining the pertinent parameter. For example, the chart can use a palette of colors wherein a given color represents a range, between minimum and maximum values, of the pertinent parameter, and the complete range of the parameter over the entire area of the chart is divided into a plurality of different ranges each having a unique designated color. In the case of a nautical chart, for instance, depth ranges can be defined, e.g., 0 to 2 feet, 2 to 4 feet, 4 to 6 feet, 6 to 8 feet, etc., and each range can be differently colored. The 0 to 2 feet range may be light yellow, the 2 to 4 feet range may be darker yellow, the 4 to 6 feet range may be yellow-green, the 6 to 8 feet range may be green, and so forth. These different depth ranges, or depth areas, will typically assume the form of generally concentric rings on the chart. Degree of transparency can further be used for designating particular characteristics or features on a chart.
Digitized nautical charts are now commonly employed by boat operators for navigational and other purposes. There are various devices capable of displaying a digitized nautical chart stored on a computer-readable storage medium. Some sonar systems used by sport fishermen, for example, are capable of displaying digitized nautical charts. Some GPS devices, computer tablets, and Smartphone devices also have such capability.
It has been proposed to provide a user-selectable palette so that the user can select key aspects to be targeted on a nautical chart. For example, the user may have the ability to highlight, with a selected color, a particular depth area as defined by user-specified minimum and maximum values bounding the depth range, or the like.
While digitized nautical charts have utility for boat operators, the accuracy of any digitized nautical chart is compromised by changing water level of the body of water of interest. That is, typically a digitized nautical chart assumes a full or standard water level for that body of water. Water level is susceptible to change, however, for various reasons. On a lake, for example, water level often changes with rainfall or lack thereof. Lakes fed by dam-controlled rivers can change water level depending on how the dam is being operated by the controlling authority. Oceans, of course, change water level constantly as the tide changes.
It has been proposed to provide a user-input water-level offset for adjusting a digitized nautical chart for changing water level. More particularly, the inventors are aware of at least one currently marketed sonar display device that can display a nautical chart having contour lines labeled with respective depth values, together with a numerical depth value determined from a sonar depth sounder mounted on the boat. The user can estimate a difference between the sonar-indicated depth at the current boat position and the depth indicated by the nautical chart at said boat position. The device includes a user-input for inputting a water-level offset value (e.g., 6 feet) based on the estimated difference. The sonar display device is then capable of changing the depth values of the contour line labels on the chart by the amount of the input water-level offset.